So update: While I was on the trails, my buddy driving behind me informed me that while my lights were on, the light that was having issues was bright red constantly(like I was braking on that side) when I turned the lights off, I acted the same way I described in my original post.
Yesterday while I was washing the car off, I swapped bulbs(the problem light with the light that's working), and the problem light was no longer acting up(now both tail lights work as they should, with the same bulbs) :roto2nuse: Not sure what that was all about, but ok. Maybe my left brake light is just picky about bulbs.